1. Field of the Invention
An apparatus consistent with the present invention relates to an apparatus for steering a vehicle, which is able to establish not only a configuration where a hand steering device and a road wheel steering device are mechanically unconnected to each other during normal traveling, but also establish the configuration these components are mechanically connected so as to facilitate mechanical steering during a system failure such as an electrical malfunction.
2. Discussion of Background Art
An apparatus for steering a vehicle with a steer-by wire method has been equipped with a connection device, which mechanically connects a mechanism for transmitting force to road wheels and a mechanism for transmitting reaction force to a hand steering device so that a reaction force is appropriately applied to the hand steering device. Japanese Published Patent Application 2003-137127 discloses related arts.
FIG. 11 is a sectional view showing a conventional apparatus for steering a vehicle with steer-by-wire method.
As shown in FIG. 11, an apparatus 100 for steering a vehicle with steer-by-wire method has two motors, one motor M1 for generating reaction force and the other motor M2 for generating steering force.
The motor M1 serves as a power source which applies a reaction force to a hand steering device such as a steering wheel (not shown). Through a mechanism 110 for transmitting reaction force, the motor M1 is mechanically connected to an input shaft 130 that is mechanically connected to the hand steering device (not shown).
The motor M2 serves as a power source for applying a force to road wheels W. Through a mechanism 140 for transmitting steering force and a shaft 150, the motor M2 is mechanically connected to an output shaft 160, which has a mechanically indirect linkage with the road wheels W.
The mechanism 110 includes a motor gear M1a, a large gear 111a, a ring gear 111, a plurality of planetary gears 112, a sun gear 113 and the input shaft 130. The large gear 111a is engaged with the motor gear M1a. The ring gear 111 is integrally formed inside the large gear 111a. The planetary gears 112 are engaged with the ring gear 111. The sun gear 113 is engaged with the planetary gears 112. The input shaft 130 rotates in agreement with the sun gear 113. A planetary gear mechanism 114, which is composed of the sun gear 113, ring gear 111, planetary gears 112 and a planetary carrier 146 that rotatably supports the planetary gears 112, is disposed in the mechanism 110.
The mechanism 140 for transmitting steering force includes a worm 141, a worm wheel 142, an intermediate gear 144, an output gear 145, the planetary carrier 146 and the shaft 150. The worm 141 is attached to a shaft of the motor M2. The worm wheel 142, which is engaged with the worm 141, rotates at a reduced speed. The intermediate gear 144 is attached to a shaft 143 to which the worm wheel 142 is also attached. The output gear 145 is engaged with the intermediate gear 144. The planetary carrier 146 is secured to the output gear 145. An end portion of the shaft 150 is secured to the planetary carrier 146.
An electromagnetic solenoid 181, which activates a lock mechanism 180 so as to lock the ring gear 111 relative to a housing 170, is attached to the housing 170. The electromagnetic solenoid 181 applies an electromagnetic force to a plunger 181a so that the plunger 181a is selectively extruded to engage with and restored to disengage from one of a plurality of engagement grooves 111b, which are formed in a top surface of the ring gear 111.
Because the plunger 181a is drawn into the electromagnetic solenoid 181 while it is energized (normal state), the plunger 181a is disengaged from an engagement groove 111b. In this way, the ring gear 111 is permitted to freely rotate. In this case, the input shaft 130 and sun gear 113, which lie at a closer side of the hand steering device, rotate independent of the output shaft 160, the rotational shaft 143 and the planetary carrier 146, which lie at a closer side of the road wheels W. This configuration is called a disconnection state, in which a rotational force generated by the motor M1 of the mechanism 110 that rotates according to an amount of manipulation applied to the hand steering device and the speed of a vehicle is transmitted to the input shaft 130 by an electronic control unit (not shown). On the other hand, a rotational force, which is produced by the motor M2 of the mechanism 140 that rotates according to an amount of manipulation applied to the hand steering device by the electronic control unit, is transmitted to the output shaft 160. In this way, an angle of steered road wheel W is controlled without a mechanical linkage with the hand steering device. A pinion 210 in a steering gear box 200 rotates in unison with the output shaft 160 via a universal joint 400. The pinion 210 engages with a rack 320 with a predetermined gear ratio, which moves in unison with tie rods 300 and the road wheels W.
FIG. 12 is a diagram illustrating characteristic curves for steering angle ratio during normal and failure modes, which are achieved by a conventional apparatus for steering a vehicle with steer-by-wire method.
As shown in FIG. 12, the hand steering device of the apparatus 100 for steering a vehicle with steer-by-wire method is adapted to have a steering angle ratio, which is defined as the ratio of an angular displacement of the input shaft 130 to an angular displacement of steered road wheels W. When the speed of a vehicle is higher, the ratio takes a greater value so as to provide less sensitive response to the hand steering device. In contrast, when the speed is lower, the ratio takes a smaller value to provide more sensitive response.
When the apparatus 100 encounters a failure due to an electrical malfunction of the motor M2 for generating steering force, for example, the ring gear 111 falls in a locked condition, because the electromagnetic solenoid 181 is energized. In such a failure, it is impossible that the sun gear 113 and the planetary carrier 146 independently rotate. As the planetary carrier 146 interlocks with the sun gear 113, the input shaft 130 and the output shaft 160 are mechanically connected, so that a rotation of the input shaft 130 is transmitted to the output shaft 160.
Japanese Published Patent Application 2003-137127 discloses the conventional apparatus 100 as shown in FIG. 11. In a case of failure due to a malfunction of the motor M2, because the mechanism 110 and the mechanism 140 are connected in a clutch-like fashion via the planetary gear 112, the rotation of a hand steering device is accordingly transmitted to the mechanism 140 with a predetermined gear ratio.
When the apparatus 100 is in a normal mode on the other hand, the electronic control unit controls the motors M1 and M2 so that an appropriate manipulation of the hand steering device can be achieved.
However, when the apparatus 100 encounters a failure, a characteristic curve K1 for a failure mode is mechanically set to a constant value (constant steering angle ratio), so that a control range R of the steering angle ratio disappears. Because a difference L1 in steering angle ratio between a failure mode and a normal mode, for which a steering angle ratio can be obtained from a characteristic curve J1, results in a greater value, it induces a sudden change in a response for manipulation of a hand steering device. In this way, a driver experiences uneasy feeling in manipulating the hand steering device.
Therefore, it is preferred that an apparatus for steering a vehicle is structurally robust enough to provide the driver with comfortable feeling for manipulation of a hand steering device, even if the apparatus falls in a failure due to a malfunction of a steering motor.